It seems too simple and the users will not get the other available block placement policies and its usage. For example in HDFS-7541, it introduced a new policy BlockPlacementPolicyWithUpgradeDomain. This policy can be good used in rolling upgrade opeations.

In addition, I don't find other documentations that concretely introducing the existing placement policies. This JIRA foucus on making a improvement for this in file hdfs-default.xml.